2014/01/27

back to the beginning ... context

I gather that Thor (aka Peter Rodgers) approves of my Back To The Beginning series. Let me tell you, it is hard going and not a post goes by without me making a couple more assumptions than I wanted and creating a couple more loose ends that I need to tie up. It is mostly context, ROC/NetKernel is what I have been breathing in the past six years. When I look at other evolutions in the IT field (yes, I do) they look as alien to me as ROC/NetKernel might look to you. It's all a matter of point-of-view, of context.This brings me to a question I got about my last post. Actually, two questions.

The first one was : "Why are you not showing a lot more code ? Things remain at the child's play stage without it, a real application has a lot more code !". While I disagree with the statement, that is a good observation that deserves an answer.I'm not easily offended and I had a very good email discussion with the person that made the observation. Feel free to contact me if you have questions too !

ROC development is not code-centric. It is about constructing small components (endpoints) that you can compose together and constrain as needed. If you know about shell scripting, you are already familiar with this concept. A lot of components (for example, I used an XSLT transformation in an earlier post) are already available.

The components should be made such that they do one task, one task only. For myself I use the following rule-of-thumb ... if a component requires more than 200 lines of source code, comments and logging included, I have not been thinking things through enough.

Tony Butterfield gave me the following thought exercise to come to grips with it. Would the Linux ls command be a good candidate for a ROC component ? The answer is ... no. The core of the command (directory listing) is fine but it has too many formatting and filtering options. By putting those options in the same component you would take away the chance for the ROC system to optimize them. They should have been in different components.

So the reason there hasn't been a lot of code in this series shows the reality of developing in ROC/NetKernel, it is not me trying to avoid complexity.

The second question was : "What is this aContext-thingie in your Java code ?". Ah, right, oops, I didn't actually discuss coding in the ROC world at all yet, did I ?

Well, to start, if it runs in the JVM, you can potentially use it to ROC code in. In practice, I find that a combination of Java and Groovy is ideal. Note that I wasn't formally trained in either. Note that I am pretty proficient in Python (and Jython is an option in NetKernel). However, if 200 lines are all I'm going to write for a given component, I'm not going to require wizardry in any language, right ? So I decided to use Java for writing new components (since NetKernel was developed in it, it is closest to the metal) and I use Groovy as my composition language.

I am quite serious, I can't stand up to any seasoned J2EE developer in a "pure" Java coding challenge and I have great respect for their skills. However, if I'm allowed to use ROC/NetKernel to solve the problem I will go toe to toe with the best.Writing a new component always follows these four steps :

What have I promised that I will provide ?

What do I need in order deliver my promise ?

Add value (the thing that makes this special).

Return a response.

When a request of an endpoint is made, you are handed the context of that request to take those steps. This context allows you to pull in the arguments to the request, for example :String theargument = context.source("arg:theargument",String.class);It allows you to create new (sub)requests in order to add value, for example :INKFRequest subrequest = context.createRequest("active:somecomponent");subrequest.addArgumentByValue("theargument", theargument);String theresult = (String)context.issueRequest(subrequest);And finally it allows you to return a response :

context.createResponseFrom(theresult);

When using Java you are given slightly more control over things, however the HelloWorldAccessor with the onSource method from the introduction module is a good starting point, we'll discuss different types of endpoints and verbs in a later post (loose ends again, I know). The same thing in Groovy would look like this :import org.netkernel.layer0.nkf.*;import org.netkernel.layer0.representation.*import org.netkernel.layer0.representation.impl.*;// contextINKFRequestContext aContext = (INKFRequestContext)context;//

aContext.createResponseFrom("Hello World");Due to the way a Groovy program is instantiated you are dropped straight into the equivalent of the onSource method in a Java accessor. Also, the assignment of context to aContext is strictly speaking not necessary, it is a coding practice that allows me to see things correctly in my editor (Eclipse). In any of the available scripting languages you'll always have context available.

So ... why are these two (the Java accessor in the introduction module and the Groovy program above) good starting points but actually bad components ? Because they don't add value, the response is a static string, I could just as well - and did in the introduction module - define a literal resource.Food for thought, no ?